1. Field of the Invention
The present invention relates to a heat sensitive composition applicable widely as a heat sensitive recording material, and a planographic printing plate precursor having a negative recording layer comprising the composition and inscribable at high sensitivity by infrared laser.
Further, the present invention relates to a planographic printing plate precursor inscribable by infrared laser, more specifically, to a planographic printing plate precursor which can form a planographic printing plate excellent in recording sensitivity.
2. Description of the Related Art
Recent development in laser is remarkable, and particularly, in solid laser and semiconductor laser which emit a light having a range from a near infrared ray to infrared ray, and progresses thereof have been made in high output and decrease in size. Therefore, these lasers are very useful as a light source for exposure in direct plate making from digital data from a computer and the like.
A negative planographic printing plate material used for infrared laser such as those described above as the light source is a planographic printing plate material having a photosensitive layer containing an infrared ray absorbing agent or light-heat converting agent, a polymerization initiator which generates a radical by the action of light or heat, and a polymerizable compound.
Usually, such a negative image recording material utilizes a recording method in which a polymerization reaction is caused by using a radical which is generated by the action of light or heat, as the initiator, and exposed portions of a recording layer is harden to form an image portion. An image forming property of the negative image forming material is low as compared with those of a positive image forming material which cause solubilization of a recording layer due to energy of infrared laser irradiation. Therefore, in general, heating treatment before a development process is proceeded for the negative image forming material in order to form a strong image portion by promoting a hardening reaction in polymerization. As the negative image recording material in which such post heating treatment is conducted, for example, recording materials comprised of a resol resin, novolak resin, infrared absorber, and acid generator are described in U.S. Pat. No. 5,340,699 and the like.
However, in such a negative image recording material, heating treatment at 140 to 200° C. for 50 to 120 seconds after exposure to laser light is required. Therefore, a large scale apparatus and energy is required for the heating treatment after exposure.
Further, when an aluminum substrate is used, energy of an infrared laser irradiation is diffused into the substrate having high heat conductivity, and the energy is not utilized for initiation and promotion for a polymerization reaction to form images, consequently, sufficient sensitivity is not obtained.
Further, Japanese Patent Application Publication (JP-B) No. 7-103171 disclosed a recording material requiring no heating treatment after image-wise exposure, the material being comprised of a cyanine coloring material having a specific structure, an iodonium salt and an addition-polymerizable compound having an ethylenically unsaturated double bond. However, this image recording material has a problem such that polymerization inhibition is caused by oxygen in the air at the time of a polymerization reaction, and sufficient sensitivity is not obtained. Furthermore, Japanese Patent Application Laid-Open (JP-A) No. 8-108621 disclosed that an image recording media containing a thermal polymerizable resin, and an organic peroxide or azobisnitrile-based compound both of which are a generally used as a heat polymerization initiator. However, any image recording sensitivity thereof is 200 mJ/cm2 or more, and therefore, preheat treatment in an exposure process is required in order to improve the sensitivity. As described above, in the present condition, high sensitivity practically required cannot be achieved.
Particularly, when a recording layer of heat mode polymerization system is used, it may use an initiator having a lower decomposition temperature and cause polymerization at lower energy, in order to improve the sensitivity. However, when an initiator having lower decomposition temperature is simply and randomly selected as the initiator, problems such as generation of pollutions in non-image portions may occur, since stability thereof may decrease.